Built-in settings dialog opened from a panel gear button

- New spawn_settings action + '≡ Inställningar' button in the default
  panel (≡ instead of ⚙ which has ambiguous character width): opens a
  singleton settings window
- Rows: theme (cycles built-in + themes/*.toml), panel (cycles
  panels/*.toml), shadow and scrollbar toggles — keyboard (arrows/
  Enter/Esc) and mouse (click selects, click again activates)
- Shows where config.toml, themes/ and panels/ live
- Changes apply immediately and persist to config.toml via toml_edit;
  the hot-reload watcher picks up the file writes for panels
- README and --help updated with switcher/snap/copy-mode/theme docs
- Integration suite: 30/30 (gear click, dialog opens, shadow toggle
  persisted to config.toml)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

panels/topbar.toml

@@ -7,6 +7,13 @@ action = { type = "spawn_terminal" }
 label = "mouse-test"
 action = { type = "run_program", command = "testapps/mouse-test/target/release/mouse-test" }
 
+# Inbyggda inställningar (tema/panel/skugga/scrollbar).
+# ≡ används i stället för ⚙ eftersom kugghjuls-emojin har tvetydig
+# teckenbredd och förskjuter panelraden i vissa terminaler.
+[[item]]
+label = "≡ Inställningar"
+action = { type = "spawn_settings" }
+
 [[item]]
 label = "Exit"
 action = { type = "exit" }